1. Field of the Invention
The present invention relates to a visible light communication system, and more particularly, to a method and an apparatus for processing incoming signals in a visible light communication receiver using a plurality of photo-detectors.
2. Description of the Related Art
Recently, with improvement in the luminance efficiency of Light Emitting Diodes (LEDs) and a price drop of the LEDs, the use of LEDs has widely spread in general lighting markets, including the markets of fluorescent lamps and incandescent lamps, as well as specific lighting markets, including the markets of portable devices, displays, automobiles, traffic lights, billboards, etc.
Recently, various factors (e.g., the exhaustion of radio frequency (RF) band frequencies, potential crosstalk between several wireless communication technologies, an increasing demand for communication security, and the advent of an ultra-high speed ubiquitous communication environment based on 4G wireless technologies) have increased interest in radio over fiber technologies complementary to RF technologies. Consequently, research on visible light wireless communication employing visible light LEDs is now in progress by many enterprises and laboratories.
Fluorescent lamps and incandescent lamps currently being used for lighting in homes, offices, and public places are expected to be replaced by LEDs having higher performance and longer life. If current applied to an LED used for lighting is modulated, the LED for lighting can be utilized as a light source for communication. That is, it is possible to provide broadcasting and data transmission only through an LED for lighting without adding a light source.
As such, visible light communication for transferring information by using visible light has advantages in that it is possible to freely perform communication through widely available bandwidth without regulation, as compared with radio communication, and it is also possible to provide reliability in view of security, as well as identification of communication links. Further, in visible light communication, it is possible to simultaneously perform a communication function and a lighting function. That is, by using a general illuminator, it is possible to act as a visible communication transceiver for transmitting and receiving information simultaneously while acting as lighting.
FIGS. 1A and 1B are views schematically illustrating a visible light receiving unit of a conventional visible light communication receiver. FIG. 1A is a view schematically illustrating a visible light receiving unit using one photodiode (PD) as a photo-detector in the conventional visible light communication receiver. Referring to FIG. 1A, the visible light receiving unit of the visible light communication receiver includes a photodiode 100 for photo electrically converting received light into electrical signals, and a lens 120 for concentrating the received light to the photodiode 100. The visible light signals transmitted from a transmitter are received into a receiver, and the received visible light signals are concentrated to the photodiode 100 through the lens 120. Then, the visible light signals are converted into electrical signals by the photodiode 100 before a reception operation is performed.
FIG. 1B is a view schematically illustrating a visible light receiving unit using a plurality of photodiodes as a photo-detector in the conventional visible light communication receiver. FIG. 1B is a view illustrating a visible light receiving unit using a photodiode array with a plurality of photodiodes in a visible light communication receiver. The visible light receiving unit of the visible light communication receiver shown in FIG. 1B includes a plurality of photodiodes 131 to 139 for photo electrically converting received light into electrical signals, and a lens 140 for concentrating the received light to the photodiode array (hereinafter, referred to as “PD array”). When the PD array including the photodiodes 131 to 139 is used, a visible light which pass through the lens are received at one photodiode or a plurality of photodiodes, depending on a light-receiving angle of the visible light. The visible light communication receiver performs a function of restoring incoming signals through electrical signals output from a photodiode at which the light has been received.
The reason why a plurality of photodiodes is used in the visible light communication receiver is that there is a relationship between a size of a photodiode area and reaction speed. In general, an increase in the photodiode area increases a corresponding light-receiving amount, but reduces the operation speed of the photodiode. In addition, resistance is increased as the photodiode area gets larger, so that the increased resistance in a circuit increases a time constant. Therefore, since the width of the photodiode is in inverse proportion to the operation speed of the photodiode, if a plurality of photodiodes with small areas are disposed to have an array configuration, it is possible to anticipate the extension of the light-receiving area together with the improvement of the operation speed.
FIG. 2 is a block diagram schematically illustrating the conventional visible light communication receiver using a plurality of photodiode as a photo-detector. The conventional visible light communication receiver using a plurality of photodiodes includes a PD array 250 which includes a plurality of photodiodes 200-1 to 200-n that photo electrically converts received light into electrical signals, a summer 210 for summing up the converted electrical signals, a demodulator 220 for demodulating incoming signals, and a decoder 230 for correcting errors by an error correction code.
The visible light signals received by the visible light communication receiver is photo electrically converted by the photodiodes 200-1 to 200-n, and then output values of each of the photodiodes 200-1 to 200-n are summed up by the summer 210.
FIG. 3 is a flowchart illustrating a reception operation of the conventional visible light communication receiver using a plurality of photodiodes as a photo-detector. The reception operation of the conventional visible light communication receiver will be described with reference to FIG. 3. In step 310, the visible light communication receiver receives visible light signals from a visible light communication transmitter. In step 320, the visible light signals are received at the PD array 250 of the visible light communication receiver, and then output values of each of the photodiodes on the PD array 250 are summed up by the summer 210. In step 330, the visible light communication receiver demodulates the summed-up signals. In step 340, the visible light communication receiver performs a decoding operation. In step 350, it is determined if the reception operation is terminated. As a result of the determination in step 350, when the reception operation is not terminated, the process returns to step 310, in which the reception operation is repeatedly performed. Otherwise, the reception operation is terminated.
Meanwhile, the visible light communication using a free space propagation scheme operates in an environment different from that of the optical communication scheme using optical fibers. In the optical communication scheme, signals are transferred using optical fibers, so that transmitted signals are received by a receiver almost without loss. However, in the visible light communication using a free space propagation scheme, visible light signals with information illuminates wide free space during transmission, so that the visible light communication receiver receives only a part of signals transmitted from the visible light communication transmitter. To solve this problem, the visible light communication receiver employs additional components, such as a lens, for increasing the light-receiving amount.
Further, as described in FIGS. 1 and 2, a scheme for disposing a plurality of photo-detectors in an array configuration may be employed, in order to improve the performance of the receiver in the visible light communication. In this case, it is possible to anticipate extension of the whole light-receiving area together with improvement of operation speed. In the case of using the above-described scheme, an area of each of the photo-detectors constituting a photo-detector array is in inverse proportion to operation speed. Therefore, as the area of the photo-detector gets larger, the amount of receivable light increases, which increases strengths of output electrical signals while decreasing the reaction speed of the detector. In other words, when the speed of change of the transmitted light by increase of the transmission speed is beyond the operation limit of the photo-detectors, the normal reception operation may not be performed. To overcome the above-described problem, there has been proposed a scheme of using an array including photo-detectors with small areas. However, in this case, an increase in complexity relatively increases cost.
Therefore, a method for restoring visible light signals with reliability is needed.